Finally, the J04C metal pads arrived. I was considering sanding the rotors with fine grit paper as some advised but looking at them under a magnifying glass the rotor surface area appeared good. I decided the fewer variables changed for the next test run the better. I simply changed out the stock fiber J02A pads with the metal J04C pads. Side by side, the metal pad is not only a more thermally conductive braking surface but also a much beefier pad including an additional silver metal plate that appears designed to conduct heat outward to the fins where the air flows.
It was also good to see that Shimano made them backwards compatible with the F03C pads I had been running on my Santa Cruz Tallboy XTR brakes. I am not certain if the F03C is forwards compatible with the BR-RS785 Road Disc. Here is the compatability table from the back of the packaging.
Road Disc 